There have been a number of proposals for determining the pressure of a vehicle tyre whilst it passes over an array of sensors. These may rely on the assumption that the contact pressure is an approximation of the tyre inflation pressure. The contact area of a tyre with a surface can be taken as the weight borne by the tyre, divided by the tyre inflation pressure. As the tyre inflation decreases, the contact area increases, although the relationship depends on the structural properties of the tyre. For example, at very low inflation pressures the reinforced sidewalls of the tyre will take a significant portion of the load directly. An alternative approach relies on investigating the lateral profile of the tyre footprint. If a tyre is over-inflated, there will be a narrow, relatively sharp profile. If a tyre is under-inflated, there will be a wider, relatively flat profile. Examples of such systems are disclosed in WO 00111442, EP 0545641 and EP 0656269.
In some cases there may be a two dimensional array of sensors, and in others there may be a linear array of sensors. In either case, outputs from the sensors are sampled at intervals as the tyre passes over. These outputs may be used to indicate the shape of the tyre footprint. In the case of a linear array of sensors, or a two dimensional array of limited extent in the direction of travel, the shape of the tyre footprint is obtained indirectly. A row of sensors extending across the tyre will always stay in contact with the same portion on the circumference of the tyre. That portion's position in the tyre footprint will alter, and the length of the line of contact will increase from initial contact, and then decrease until contact ceases. The shape of the tyre footprint is extrapolated from the sensor data.
Most known systems ignore the effects of the tread cut into the surface of the tyre. There are tread gaps where there is no contact between the tyre and the sensor array, and it is not possible to use the data to determine, for example, the area of contact between the tyre and the surface within the footprint of the tyre. This can lead to inaccuracies in the measurement of such quantities as tyre pressure, load on a wheel, axle load and vehicle weight.
In WO 2006/003467 there is disclosed a system which uses sensors of sufficiently high resolution to detect tread gaps on a tyre, so that is possible to estimate the extent of tread coverage and to determine the tyre pressure using sensors which are fully in contact with the tyre rubber, without the area of contact being affected by a tread gap. The system uses a linear array of sensors, and the output data is from a line of contact at a circumferential position on the tyre. Some calculations are made based on the areas of the individual sensors, and for some purposes there is provided an estimate of the area of the tyre footprint.
U.S. Pat. No. 6,823,728 discloses a system for estimating tyre pressure in which there is provided a weight sensor and, overlying that, an area sensor. The area sensor estimates the contact area directly. This may be by means of: a capacitive sensor; or line switches to determine the length and width of the tyre contact area; or a linear switch array whose outputs are sampled and used together with vehicle speed, detected using a pair of line switches, to estimate the tyre footprint area. The use of the switch array method is said to provide a more accurate estimation of the area than if using only the length and width. With the total weight known, the tyre pressure can be calculated by dividing the weight borne by the wheel, by the tyre footprint area.